Nucleotide sequence of the gag-pol region of the HIV-1 virus, encoding for a 99 amino acid long protease was synthesized in the pol open reading frame. After additions of the translational initiation and termination codons to the appropriate ends, the protease gene was inserted into a high expression vector which was then used to transform E. coli cells. The recombinant clones beuring the correct coding sequence of 297bp were analyzed for expression of the protease gene product using specific antibodies. The product was detected in Western blots as a single 11.5kd protein. After selecting the optimum conditions for expression of this gene and solubilization of the protein, the specific proteolytic activity was proven by its capacity to cleave correctly a synthetic nonapeptide, corresponding to the HlV-1 P17-P24 cleavage site. This cleavege, together with others, is necessary for the processing of the gag-pol precursor, and thus to the retroviral life cycle. Specific inhibitors of the retroviral protease, now being sought, might be effective anti-retroviral therapeutic agents.